The present invention relates to an I/O (Input/Output) control system and method, and more particularly to a start control system and method for an input/output device, the start control being off-line processed from an I/O control unit prior to a data transfer process.
A magnetic tape of a conventional magnetic tape unit provided with a vacuum column, which is essential for a high speed process, can be started at a high speed. However, a cartridge type magnetic tape unit is not provided with such a vacuum column to make it light in weight and small in size. In view of this, the start process is off-line processed and thereafter, when a data transfer becomes possible, the magnetic tape unit is reconnected to the I/O control unit. Thus, it is possible to execute in parallel both data transfer and start process of different magnetic tape units, so that the throughput of the whole system is improved. The above concept has been applied to such devices as magnetic disk units.
According to Japanese patent application No. 59-10074 filed in the name of the same assignee as the present application (corresponding to U.S. Pat. No. 4,754,399 "Input Output Control", issued Jun. 28, 1988), if a data transfer path is unoccupied, a start request is issued to the I/O device to or from which data transfer is desired. At the end of the start process, the I/O control unit is reconnected. If at the end of the start process, another I/O device is under data transfer and the data transfer path is occupied, then it is necessary for the I/O device to follow a stop/rewind/restart process. Thus, this I/O control system is not always efficient.
Japanese Patent Unexamined Publication No. 57-159324, (corresponding with U.S. Pat. No. 4,423,480), relates to a system in which an exclusive path for a control signal transfer is provided, through which a start request is issued to an I/O device while another I/O device is under data transfer. Described therein is an operation in which the time required for starting and for data transfer to or from an I/O device are equal. In this case, for example, if a data transfer of an I/O device A starts at a time when a start request is issued to another I/O device B, then the start process of the I/O device B completes just when the data transfer of the I/O device A terminates. However, in general, it is common that the data transfer rate and the time required for starting an I/O device differ from each other. Thus, it is impossible for I/O devices to incessantly use the data transfer path. An effective use of the data transfer path has not been possible in the conventional I/O control system.